Does green algae hold a genetic clue to the origin of the sexes?

The origin of how we became men and women - how species evolved to have physically distinct males and females - may be demystified, say plant biologists, after they identified a master gene in control of sex and mating among green algae.

The gene discovered in the Volvox carteri algae was found to be responsible for the distinct physical differences between the males and females in the multicellular species.

The gene discovered in the Volvox carteri algae was found to be responsible for the distinct physical differences between the males and females in the multicellular species.

This could also "provide a possible blueprint for how sexes in other multicellular organisms like plants and animals may have originated," say the researchers.

The contrast between the two types of reproductive cells (known by biologists as the gametes, which fuse during fertilization) are obvious: it is the difference between sperm and eggs.

"However," say the researchers, "the evolutionary origins of male and female sexes are unclear because the distant unicellular relatives of plants, animals and other multicellular species generally don't have distinct sexes, but instead have mating types - a system in which gametes of one mating type can only fuse with those with a different mating type, but the cells of each mating type are indistinguishable from each other in size and morphology."

Gene evolved from single-celled species

The authors have published the results of their work on Volvox in the open-access journal PLOS Biology.

The sexes of that multicellular alga were found to be derived from the mating types of its single-celled relative, a species known as Chlamydomonas reinhardtii.

A gene was central to the determination of sperm versus egg - the laboratory work clarified previous theoretical work to find that "a single ancestral mating locus gene, MID, evolved from its role in determining mating type in C. reinhardtii to determine either spermatogenesis or oogenesis in V. carteri."

How do the findings relate to more complex beings? The authors say the algae provide an "excellent model for investigating the evolution of sexual dimorphism" because of the way they show "progression from unicellular species to multicellular forms with increasing size and cell-type specialization."

Gender reassignment

The biologists' findings are highly scientific and involved meticulous laboratory work, but essentially they gender-swapped the algae through genetic manipulation.

The team - led by Dr. James Umen, associate member of the renewable fuels institute at the Danforth Plant Science Center in St. Louis, MO - "was able to convert what would have been egg cells into packets of functional sperm cells" in the genetically female Volvox.

Conversely, the team also forced the genetic males "to develop with functional eggs in place of their sperm packets."

This was all achieved through the manipulation of a single gene called MID, which the team identifies as the "master regulatory gene for sexes and mating types" in this group of green algae.

The results of the work "hint that a similar evolutionary scenario may underlie the origin of sexes in animals, plants and other multicellular lineages," say the plant scientists.

In addition to the contribution to evolutionary theory, the findings could give practical help to scientists working on the development of biofuels and other biotechnological applications, where, says Dr. Umen, "sexual reproduction in most algal species is poorly understood."

'Remarkable' findings

In an editorial published in the same issue of the journal, UK-based medical and molecular geneticist Roland G. Roberts says the researchers were able not only able to show that the genetic switch was a male-versus-female one, but also one that could determine hermaphrodites.

With some genetic details, he writes: "Remarkably, the authors were able to show that this male/female switch is tunable - when they used a weaker version of the Volvox carteri MID RNAi hairpin, the partial depletion of Volvox carteri MID resulted in hermaphrodites that contained both pseudo-male sperm packets and pseudo-female eggs."

The two species of algae studied, which are "separated by 200 million years of evolution," adds Dr. Roberts, "couldn't be more different in their appearance or sexual proclivities." Yet, he writes, a single gene not only determined the sexes in both species, but drove the dimorphism between the sexes, and the "considerable divergence" in the way the two species develop sexually.

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